The invention relates to optical ranging apparatus for measuring the range (distance) to a source of electromagnetic radiation of infrared and smaller wavelengths.
In scientific, industrial, and military fields there is often the need to passively measure the range to a source of electromagnetic radiation. For example, it is known that the range to a scanning source of radio frequency emissions, e.g., surveillance radar can be determined by passive (non-radiating) ranging techniques in which the radiation is received at slightly different angles by first and second sensors spaced apart along an axis perpendicular to the direction of arrival of the source emissions and the range is derived from the timing information.
The invention described hereinafter pertains to the type of ranging technique involving a passive system and a nonscanning emitter, hence passive ranging on nonscanning emitters (PRONSE). More specifically, the invention is concerned with emitters of electromagnetic radiation in the optical range, namely infrared and smaller wavelengths.
Another related technique is used in determining the range to a source of electromagnetic radiation in the light spectrum. Optical ranging apparatus have been heretofore developed, as for example, disclosed in U.S. Pat. No. 4,178,098, in which the distant object that is emitting (or reflecting) a source of light, is directed onto first and second optical/photodetector assemblies mounted in juxtaposition and separated by a distance d. Relative positional parallax causes the image to be focused on slightly different positions separated by an offset, i.e., the image of one optical channel is slightly offset on the associated detector array as compared to the image focused on the detector array of the juxtaposed optical channel. This offset (.DELTA.x) is known to be related to the distance or range by a formula that includes the focal length of the optics, the distance d separating the centerlines of the optics, and the field of view of the optics. Furthermore, it is known that the offset .DELTA.x may be measured by comparing the output signals from the detectors of the first and second arrays, and from the measured offset .DELTA.x, the range can be computed.
In electro-optical ranging apparatus and method of this type, however, the accuracy of the measurement depends upon the capability of the system to measure the offset .DELTA.x. For systems in which the separation distance d between the centerline of the optics is relatively small, compared to the range to the object or emitter of electromagnetic radiation, the optical images that are projected onto the photodetector arrays are very closely overlapping. In other words, the image focussed on a first of the side-by-side photodetector arrays falls nearly at the same corresponding position as the image on the second array. Thus, the separation or offset .DELTA.x between the images is exceedingly small and the comparison of the detector output signals to extract this very small offset .DELTA.x becomes crucial to the effectiveness of the equipment.